1. Field of the Invention
The present invention relates generally to display apparatus for indicating the deviation of process value from set point in a controlled process loop and more particularly to apparatus for providing scan-line type indication of such deviations for a plurality of controlled process loops.
2. Description of the Prior Art
An industrial process includes numerous process variables, such as pressures, temperatures, flow rates and levels, whose values are a function of the settings or dynamics of various control devices, such as valves and pumps. Proper performance of such a process requires that such process variables and/or the relationship (ratio, etc.) of such process variables to one another or to the other factors, such variable values and relationships being referred to as process values, be maintained at predetermined values, referred to as set points. Thus, such processes include process control systems in which the process variables are monitored so that the process values can be determined, such process values are compared to the appropriate set point, and the appropriate control devices are adjusted, such as by resetting a valve or the speed of a pump, to correct any deviation of such process values from set point.
In an automatic process control system, such control of process variable values is accomplished through one or more control loops. In a control loop, the values of one or more of the process variables (loop variables) are indicated as electrical signals (analog signals) generated by transducers associated with such variables and/or as contact closures or openings (discrete signals) generated by switches associated with such variables. These values are supplied to a controller as the loop data input. The controller, which ordinarily includes either an analog or a digital computer, is programmed to act on such loop data input in the appropriate fashion, including, if necessary, determining the relationship of one process variable to another or to a constant programmed into the computer, so as to determine the loop process value. This loop process value is then compared to the loop set point so as to determine the deviation between the loop process value and set point. The set point is stored in the controller either as a voltage level (in the case of an analog controller) or as a digital value in memory (in the case of a digital controller). The controller then provides one or more output signals in the form of a continuous electrical signal (analog output) and/or as contact openings and closures (discrete output). The controller is programmed in accordance with appropriate control strategy such that the output signals adjust the control devices associated with the loop variables so as to decrease the deviation between the loop process variable and the loop set point (loop deviation).
During ordinary operation, an automatic control system should be capable of controlling an extremely large process with a tremendous number of variables with essentially no human intervention; i.e., once the controllers for all the loops have been properly programmed to maintain the various loop process values at their respective loop set points, such programming being accomplished through a programming interface between each controller and a programmer, the controllers should be able to adjust the control devices to keep the loop deviations at approximately zero without further adjustment. In this regard, the dynamics of the process and control devices often make perfect control of the variables impossible; i.e., there will often be some deviation between each loop process value and the corresponding loop set point. As a result, a certain amount of deviation between a loop process value and a loop set point is acceptable. Thus, the control strategy for a loop is designed to keep the deviation for that loop within the acceptable limits without human intervention.
Certain conditions may arise, however, in which the controller of a loop is unable to maintain the loop deviation within acceptable limits. Such excessive loop deviation not only may prevent the process from operating optimally, but also may cause damage to the process and endanger the safety of the operators. Thus, most automatic control systems include an indicator interface for providing a visible indication of the various loop deviations. Such indicator interfaces are sometimes coupled with an alarm interface that provide special visible and/or audible signals whenever an alarm condition, which may be an excessive loop deviation, exists. By virtue of such interfaces, an operator is able to take extraordinary measures to correct the alarm condition.
A well-known indicator interface of the prior art has been the scan-line indicator. Such scan-line indicators have been used in conjunction with automatic control systems that have a large number of loops and include a separate analog controller for each loop. Each controller includes a fixed, horizontal, transparent, usually green, bar disposed over a scale that is adjustable with respect to such bar. The bar has an opaque, horizontal hairline extending along its axis, such line representing set point. The indicator interface of each controller further includes a horizontal, vertically movable pointer whose vertical position is a function of process value for the loop of the controller. When such loop process value is equal to the loop set point, the pointer of the controller indicator interface will be disposed either in front of or behind the bar of such interface in vertical alignment with the hairline of such bar. As the loop process value deviates from the loop set point, the pointer will move above or below the hairline, depending on the direction of deviation. The amount of spacing between the pointer and the hairline is dependent on the percentage of the loop deviation with respect to set point ("percentage of loop deviation"). Generally speaking, if the pointer is within the width of the bar, although spaced from the hairline, the percentage of the loop deviation can be regarded as small, but if the pointer is spaced above or below the bar, the percentage of loop deviation can be regarded as large.
By placing several of such controllers side-by-side such that the bars of the indicator interfaces of such controllers are aligned with one another, information pertaining to the loop deviation of several loops can be quickly observed by a horizontal scan of the bars of each controller. If the percentage of loop deviation of each controller is small, the needle of the indicator interface of each controller will be observed within the width of the green bars during such scan. If the percentage of loop deviation of one of the controllers is large, the observer will be alerted to such large deviation during the scan by the absence of the needle of the indicator interface of such controller from the width of the green bar of such interface.
While such prior art indicator interfaces are capable of apprising an observer that a percentage of loop deviation is large or small, they do not provide any readily observable information with regard to the significance of such deviation, e.g., they do not specifically indicate whether an alarm condition exists. In this regard, it should be noted that while, for some process variables, such as temperatures and flow rates, small deviations, e.g. two percent, are critical and may constitute an alarm condition, for other process variables, such as level, large deviation, e.g., 30 to 40 percent, are acceptable. As a result, the operator may either fail to timely acknowledge an alarm condition or unnecessarily react where no alarm conditions exists.